Hydraulic clutch wear warning device



Och 22, 1968 Fe. G. BUCHANAN ETAL 3,40698@ HYDRAULIC CLUTCH WEAR WARNINGDEVICE Filed Dec. 13, 1956 United States Patent O "ice 3,406,800HYDRAULIC CLUTCH WEAR WARNING DEVICE Richard G. Buchanan, Des Plaines,and Frank Spinuzza, Park Ridge, Ill., assignors to Tel Life Corporation,Park Ridge, Ill., a corporation of Illinois Continuation-impart ofapplication Ser. No. 472,894, July 19, 1965. This application Dec. 13,1966, Ser. No. 616,143

7 Claims. (Cl. 192-91) ABSTRACT OF THE DISCLOSURE Device for indicatingexcessive clutch wear in a hydraulic clutch by detecting excessiveforces exerted by clutch springs upon the slave piston, and an electricdevice on the slave cylinder assembly for sensing such forces.

CROSS REFERENCE TO RELATED APPLICATIONS This application is acontinuation-impart of prior copending application Ser. No. 472,894filed July 19, 1965, and now abandoned.

BACKGROUND OF THE INVENTION The present invention relates generally tohydraulic clutch systems and more particularly to a device for detectingexcessive clutch disc wear in a hydraulic clutch system of the typefinding particular utility in automotive vehicles.

Operation of conventional mechanical clutches in medium and heavy trucksinduces considerable driver fatigue when operation has to be repeatedseveral hundred times a day. As a result, the hydraulic clutch, whichrequires far less physical effort, has gained great popularity in recentyears. Briefly, an hydraulic clutch differs from the conventional clutchin that an hydraulic actuating system is interposed between the clutchpedal operated by the driver and a .throw-out means for effectingdisengagement of the clutch. The usual hydraulic clutch system includesat one terminal end the fly-wheel which is attached to the crankshaft ofthe vehicles engine and the transmission pilot shaft at the other oroutput terminal end with the clutch elements including7 a friction discinterposed between the terminal ends. The friction disc is coupled tothe transmission and is sandwiched between the fly-wheel and a pressureplate which, under the influence of strong expansion-type springs,forces the friction disc against the fly-wheel so as to effect drivingengagement of the clutch to transmit power from the crank-shaft torotate the transmission pilot shaft.

The clutch is disengaged by withdrawing the pressure plate against theaction of the springs. For this purpose there are usually providedseveral fulcrumed fingers which are pinned at one endto the pressureplate, with their other end disposed in juxtaposed relation to athrowout bearing `which s slidably mounted on the drive shaft of thevehicle. To disengage the clutch, the throw-out bearing is moved againstthe fingers, and the latter move the pressure plate against the springswhich normally maintain clutch engagement. It is to effect thisthrow-out bearing movement that an hydraulic system is employed whichincludes a slave cylinder and piston of which the piston ishydraulically connected to a master cylinder whose associated piston islinked with the foot pedal operated by the driver.

A source of trouble peculiar to hydraulic clutches has been theexcessive wear of clutch discs without detection 3,406,800 Patented Oct.22, 1968 back spring. Through an appropriate linkage this also serves toprevent engagement between the throw-out bearing and the pressure platefingers. As the clutch disc wears, the pressure plate is permitted tomove ever nearer the fly-wheel until the pressure plate fingers, whichare normally spaced from the throw-out bearing, are caused to pressagainst it. This in turn results in the slave piston being urged closertoward the bottom of the slave cylinder in response to the action of thepressure plate springs. Such excessive clutch disc wear not only permitsthe clutch assembly to be damaged by the ultimate destruction of theclutch disc but also, through the great force to which the slavecylinder is subjected by the clutch springs, can cause damage to thelatter unit as well.

Itis therefore an object of this invention to provide a device whichwill provide a warning that disc Wear in hydraulic clutch system isapproaching or has become excessive.

It is another object of the invention to provide an irnproved hydraulicclutch system including 'means for indicating excessive clutch discWear.

Is is still another object of the invention to provide a self-contained,minimum force responsive switch finding particular utility in anhydraulic clutch system for detecting the presence of excessive forceurging the piston in a hydraulic cylinder toward'the bottom of thecylinder so as to forewarn of that condition and permit of timelyappropriate corrective action.

BRIEF DESCRIPTION OF THE DRAWINGS The objects of the invention thusgenerally set forth, together with other objects and ancillaryadvantages, are attained by the construction and arrangement shown byway of illustration in the accompanying drawings, in which:

FIG. 1 is a schematic representation of an hydraulic systemincorporating the present invention;

FIG. 2 is a longitudinal section through a switch of the typeincorporated in the hydraulic clutch mechanism of FIG. l and embodyingfeatures of the present invention.

While the invention is susceptible of various modifications andalternative constructions, there is shown in the drawings and willherein be described in detail one prefered embodiment. It is to beunderstood, however, that it is not thereby intended to limit theinvention to the specific form disclosed. On the contrary, it isintended to cover all modifications and alternative constructionsfalling within the spirit and scope of the invention as expressed inIthe appended claims.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to the drawings, therelis shown in FIG. 1 a schematic representationV of a typical hydraulicclutch system incorporating a preferred form of the invention. Thepurpose of this figure is to assist in the understanding of theorganization and operation of the clutch system generally, and to showthe novel and distinctive manner in which operational requirements aremet by the invention.

As shown, the clutch assembly 10 includes a fly-wheel 11 fastened by thebolts 12 on Harige 13, by means of which a driving connection with thevehicle engine is effected. A ring gear 14 carried by ythe fly-wheel 11so as to permit starting of the engine by a starter motor, not shown.

Mounted on fly-wheel 11 is pressure plate cover 15 which in theillustrative clutch assembly supports through fingers 16 (only one ofwhich is shown) a pressure plate 17. Mounted between the fly-Wheel 11and pressure plate 17 is a driven friction disc 18. An internallysplined hub 19 is secured to the friction disc 18 and serves to effect adriving connection between the disc and the complemen- 3 tally splinedend of a pilot shaft 20 of the vehicle transnission.

The clutch assembly is maintained normally engaged 1nd to this endclutch springs 21, compressed between th: pressure plate 17 and thepressure plate cover 15, force the pressure plate against the frictiondisc 1S so as to Erictionally engage it with fly-wheel 11.

For effecting disengagement of the clutch there is provided a thrown-outbearing 22 which is operative in conjunction with the fingers 16 to movethe pressure plate 17 against the action of the springs 21. The throwoutbearing is slidably supported on cylindrical guide 23 which is disposedabout the transmission pilot shaft 20. The throw-out bearing 22 is movedalong its guide 23 toward pressure plate 17 and against the inner endsof A lingers 16. The outer ends of the fingers 16 are pinned to pressureplate 17 and they are fulcrumed intermediate their ends to the pressureplate cover 15. As the throwout bearing is moved against the free oflingers 16, the fingers rock about their fulcrum and withdraw thepressure plate 17 against the force of springs 21. When this occurs, thefriction disc 18 is freed from driving engagement with fly-wheel 11 andthus the driving connection between pilot shaft 20 and crank-shaftliange 13 is interrupted.

In the illustrated clutch mechanism an hydraulic system, including amaster cylinderassembly having a slave cylinder assembly 26hydraulically coupled thereto, is interposed between the clutch pedal 24and the throwout bearing 22 so as to amplify the force applied to theclutch pedal by an operator. The master cylinder assembly 25 is of theconventional type and includes a master cylinder 27, communicatingthrough port with a fluid reservoir 29, and a master piston 30. Thelatter is mounted for reciprocation in the master cylinder 27 inresponse to movement of the clutch pedal 24. When the clutch pedal isdepressed, the piston 3G in its corresponding movement first covers theport 28 so as to define a pressure chamber 31 with the walls of mastercylinder 27. Pressure chamber 31 communicates through a pressure fluidline 32 with a similar pressure chamber 33 within the slave cylinderassembly 26 as formed by the walls of slave cylinder 34 and the head 3Sof slave piston 36. When the clutch pedal 24 is depressed, fluidpressure is applied from the pressure chamber 31 through the line 32 tothe pressure chamber 33 of the slave cylinder assembly 26, which isfixed in place by lugs 37 with respect to the clutch assembly 10, thuscausing the slave piston 36 to be forced outwardly, away from the bottomend of the slave cylinder 34.

The force of the piston 36 is applied to the throwout bearing 22 throughan appropriate linkage which as shown includes a partially threaded rod38, clevis 39 and an arm 40. Throw-out bearing 22 is provided with acollar 41 which has a peripheral recess 42 therein for receiving one endof the arm 40. The other end of arm 4f) is pinned in the clevis 39. Thearm 40 is fulcrumed on a fixed ball stud 43. The clevis 39 receives thethreaded outer end of rod 38, and the inner end of the rod 38 issmoothly rounded for seating engagement in a recess formed in the rearof the piston 36. It will be apparent that by adjusting the amount ofthreaded engagement of the rod 38 into or out of clevis 39 the effectivelength of the rod can be altered, and through the resulting angularmovement of the fulcrumed arm 40, the position of the throw-out bearing22 is adjusted. This latter affects the amount of separation between themain ends of the pressure plate fingers 16 and throw-out bearing 22 sothat normally, when the clutch is engaged, the throw-out bearing 22 isheld apart from fingers 16 such spacing being maintained by anappropriate biasing means, here shown as a pull-back spring 44 which isattached to the arm 40.

To disengage the clutch, the clutch pedal 24 is depressed, pressurizingthe fluid within pressure chambers Cil 31 and 33 lforcing the slavepiston 36 to moveoutw-ardly-away from the bottom of the slave cylinder34. Through the threaded rod 38 and clevis 39, the arm 40 is rockedabout ball stud 43 against the pull of spring 44, and the throw-outbearing 22 is moved into engagement with the free inner end of fingers16. Under the urging of the throw-out bearing 22, the lingers 16 rockabout their fulcrums, withdrawing the pressure plate'17 against theaction of the clutch biasing springs 21, and the friction disc 18 isfreed from driving engagement with the fly-Wheel 11.

So long as the clutch assembly is properly adjusted and friction disc 18is not excessively worn, operation of the hydraulic clutch system isquite satisfactory. The hydraulic system is, however, subject to seriousdamage and possible destruction when there is excessive wear ormisadjustment in the clutch. Thus, when properly adjusted, the throw-outbearing 22 is kept out of contact with fingers 16, and only normal discwear occurs upon engagement and disengagement of the clutch.Additionally, since there is normally no contact between the fingers 16and the throw-out bearing 22, the clutch springs 21 are not permitted toexert force through ythe arm 40 and rod 38 upon the slave piston 36, andthe latter is urged toward the bottom of the slave cylinder solely bypull-back springs 44.

In contrast, should the friction disc 18 become excessively thinned bywear, the fingers 16 will be rocked so that their inner endscontinuously engage the throwout bearing 22 while the clutch is engaged.Not only does this result in excessive wear of the fingers 16 and of thebearing 22, but it permits the very large force of the clutch biasingsprings 21 to be exerted, in reverse, through fingers 16, the throw-outbearing 22, the arm 40, and rod 38 against piston 36 in addition tothepredetermiued force exerted on the piston by the pull back springs44. This additionall force can be sufficient, in cases of extreme clutchdisc wear, not only to require replacement of the disc 18 but even tonecessitate replacement of the slave cylinder assembly 26.

In accordance with the invention, the reverse action of the clutchsprings is utilized to warn against excessive clutch disc wear bysensing and indicating the presence of their additional force on thepiston 36. In carrying out the method of the present invention,therefore,

means are provided for sensing when the piston 36 is being urged towardthe bottm of the cylinder 34 by the clutch springs 21. Briefly stated, apreferred form of the sensing device includes a first electrical contactmounted on a plug which is inserted in the end of the slave cylinder 34and a second contract which is so located that it tends to move towardthe first contact in response to lmovement of the piston 36 toward thebottom of the cylinder. The contacts are normally held separate by ayieldable means whose yield point is higher than the force of thebiasing spring 44 upon the piston 36 but is lower than the sum of forcesexerted upon the piston by the biasing spring 44 and the -clutch springs21. Consequently, when excessive clutch disc wear occurs and the clutchsprings exert their additional force upon the piston 36, the yieldableseparating means is overcome and the contacts engage. In essence, theswitch is a special form of a force responsive means for conditionallyconducting current, requiring for its operation a force which is greaterthan that exerted on the piston by the biasing spring 44 but less thatexerted on the piston by thebiasing spring and the clutch springs 21together.

A specific embodiment of the preferred form of the electrical device for-detecting excessive clutch disc wear is shown in FIG. 2. Seated in athreaded opening in the bottom of the slave cylinder 34 is a plug 46,preferably of metal. The plug 46 is sealingly held in the threadedopening by a threaded shank 47, although a smooth shank, press fitted ina smooth cylinder opening might also be used. Mounted on the plug 46 isa first electrical "mally open.

contact member shown as ahelical spring 51 extending a short distancebeyond the end -.of the-'plug shank 47. Means are provided forconnecting the spring 51 to a circuit outside of the cylinder and intheillustrative embodiment this is done not through the metal of the plug46 but in isolation therefrom. In particular, the plug 46 has an axialbore 48 through which there extends a hollow insulating sleeve 49. Thespring 51 extends through a substantial length of the sleeve 49 where itmakes electrical connection with a terminal 50 sealed into but extendingh'om the sleeve 49. For mechanical convenience and to insure a pressureuid seal upon mounting the insulating sleeve 49, the bore 48 Visenlarged at its inner end so as to receive a flanged portion 52 of thesleeve 49. Similarly the terminal 50 is stepped at its inner end toaccommodate the spring 51.

-Located for movement toward the first contact, the spring 51, inresponse to movement of the piston 36 toward the bottom of the cylinder34 is a second contact member, here shown as a cap 53, separated fromthe spring 51 by a yieldable means such as the helical spring 54 towhich the cap 53 may be suitably aixed. To keep the contacts 51 and 53separated, the axial length of the spring S4 is slightly greater thanthe length of the protruding end portion of spring 51 so that contact 53is normally disposed a short distance beyond the end of spring 51 thusrendering the two switch contactsnorvIn the exemplary embodiment of theinvention, with the switch.45 installed in the end of slave cylinder 34as shown in FIG. l, the slave cylinder 34 is electrically 'connectedthrough integral lugs 37 to the'negative or ground side of the vehicleselectrical system and contact 53 of the switch 45 is also connected tothe vehicle ground through contact spring 54 and plug shank 47.

In accordance with an important feature of the invention, the yieldpoint of the means for keeping the contacts 51 and 53 separated, i.e.the force required to overcome it, is higher than the force of thebiasing spring 44 upon the piston 36 but lower than the sum of forcesexerted on the piston by the biasing spring `44 and the clutch springs21. As a result, the separating means, here the spring 54, yields andthe contacts 51 and 53 engage only when the piston'36 is additionallyurged toward the bot- .tom of the cylinder by the clutch springs 21 i.e.when the clutch disc 18 has worn excessively.

Thus it is seen that the specific exampleshown in FIG. 2 represents asimple self-contained switch which will re- Y' liably sense excessiveclutch disc wear by permitting closure of its contacts when and onlywhen the piston of the slave cylinder assembly 26 is urged toward thebottom of the slave cylinder 34 by the clutch springs 21. It uwill beunderstood that while a particular type of switch has been describedwhich is particularly suitable for carrying out the invention, it `isnevertheless exemplary only. Other equivalent devices for detecting theY. presence of the additional force of the clutch springs 21 upon thepiston `34 will occur to those skilled in the switch art withoutdeparting from the invention. For example, the opening in the bottomofthe cylinder 34 might be eliminated and the tirst contact might be aleaf spring, cantilever mounted on a plug which ,is inserted through .fthe side of the cylinder near its bottom, with the spring extendingdiagonally across but closely spaced from the bottom of the cylinder.The second contact might be the bottom of the cylinder itself or aresilientV contact supported'underneath the first contact by means of ananchor `plate seated at the bottom of the cylinder and in electricalcontact therewith. The strength of the leaf spring would be such that itwould be deflected against the second contact by the piston only whenpiston is urged toward the bottom of the cylinder by the clutch springsdue to excessive clutch wear. lThe leaf spring wouldbe insulated fromthe piston as well as-from the plug on which it is mounted with aninsulated lead extending from the spring through the plug to the outsideof the cylinder. The electrical circuit would then be from the leadthrough the contacts to the cylinder.

Even the particular switch shown in FIG. 2 could be modified in severalaspects. For example, instead of a cap, the second contact 53 might beformed simply by crimping the end of the spring 54 inwardly so as tocross the outermost turn of the spring 54 diagonally, substantially inthe plane of that turn. Alternatively, the cap might be replaced by alstraight, diagonal cross bar member affixed to the end of the spring 54,as by welding. As another alternative, if the piston head 35 isconductive it may constitute the second contact 53.

In the illustrative device, a visual signal in the form of a warninglamp 56, which may be mounted on the dash board of the vehicle isemployed to indicate extensive clutch wear. The lamp 56 is conductedbetween the terminal of the switch 45 and the positive side of thevehicles electrical system.

So long as the clutch is properly adjusted and the plate 18 is notexcessively worn, the lamp 56 never lights. Thus, when the clutch pedal24 is depressed, slave piston 36 is forced away from the bottom of slavecylinder 34 so as to disengage the pressure plate 17 from friction disc18 and the contact S3 is free from engagement with the head 35 of theslave piston 36. Similarly, when the clutch pedal is not depressed, theslave piston head 35 rests against contact 53 with a force determinedsolely by pull-back spring 44. As pointed out previously, the spring 54which mounts the contact 53 is selected to have a yield point which ishigher than the force of pull-back spring 44. In actual practice adifference of two pounds between the spring constants of the springs 44and 54, with the latter spring being the stiier, has been foundetiective to insure that normally the piston head 35 will be urgedagainst the switch contact 53 with a force which is insufficient to movethat contact into engagement with the spring 51.

If, however, friction disc 18 is excessively worn or the length of therod 38 is maladjusted, it is intended that the lamp 56 should remainlighted Whenever the clutch pedal 24 is released so as to provide acontinuous warning signal that attention is required. Thus, when theclutch friction disc 18 is thinned due to excessive Wear, pressure plate17 is pressed past its normal position by the clutch springs 21 and thefree ends of fingers 16 ride continuously on the throw-out bearing 22.As soon as this condition occurs, the force exerted by the springs 21through fingers 16, throw-out bearing 22, arm 40, clevis 39 and rod 38upon piston 36 becomes sufficient, together with the force of pull-inspring 44, to overcome the oppositely applied force of switch spring 54so as to cause contacts 53 and 51 to close. With the switch 45 closed,an elec trical circuit is completed from the positive side of thevehicles electrical system, the lamp 56, the terminal 501 spring contact51, contact 53 and spring 54, and shank 47 of switch 45, through slavecylinder 34 to the negative or ground side of the vehicle electricalsystem, and the lamp lights. It will be noted that contacts 51 and 53close before the piston 36 reaches the inner limit of its travel asdetermined by the plug shank 47. The contact springs 51 and 54 can thenbe further compressed by the piston 36 in case the alarm is not heededimmediately, without damage to the cylinder 34.

Misadjustment of rod 3'8 and clevis 39 will also cause r lamp 56 toremain lit and thus to warn of an unsatisfactory clutch condition, whenthe adjustment is in such a direction as to move the bearing 22 intoconstant abutment against the fingers 16. This will generally occur whenupon initial adjustment of the clutch insufficient clearance is leftbetween the free end of the clutcl. fingers 16 and the face of throw-outbearing 22. Wher this occurs, after very little wear on the frictiondisc 18l springs 21 can exert their lforce through fin-gers 16 againstthe face of throw-out bearing 22 in the manner pre 7 viously describedto force slave piston face 35 with excessive pressure against the switch45.

From the foregoing it will be apparent that an hydraulic Iclutch wearwarning device constructed in accordance with the teachings of the`present invention is inexpensive, and it may be easily installed inclutch assemblies that are already in use simply by installing a slavecylinder appropriately incorporating a clutch wear warning deviceembodying the teaching of the present invention. Further, it is apparentthat the disclosed switch element of the warning Adevice is responsivesolely to excessive mechanical force and is unaffected by andnonresponsive to changes in lluid pressure within the clutch system,thus precluding the giving of false wear warning due to Huid pressureincrease upon actuation of the operators pedal. Yet the device iseiiective to aiford a warning not only of excessive clutch disc wear butalso of misad-justments in the clutch before such wear or misadjustmentslead to clutch failure.

We claim as our invention: 1

1. For use in an hydraulic clutch system having a slav cylinder and apiston mounted for longitudinal movement therein, said piston in itsrest position being urged towards the bottom of the cylinder by abiasing force of a iirst magnitude under normal operating conditions andby an additional force Iwhen the system is excessively worn ormisadjusted, a self-contained switch for revealing the approach ofbottoming of the piston in the cylinder under the ur-ging of said secondforce, said switch comprising in combination (a) a metal plug having (1)a threaded shank for reception in an aperture in the bottom of thecylinder, and

(2) a port extending axially through the plug to provide a passage intosaid cylinder,

(b) an insulating sleeve extending within said port,

(c) a iirst spring disposed within said sleeve and having one endextending beyond said shank,

(d) a terminal extending into said sleeve and in electrical contact withsaid ltirst spring,

(e) a second spring seated at one end in said shank in electricalcontact therewith and having its other end extending towards said pistonbeyond said shank and said one end of said irst spring.

(f) a metal cap supported on the other end of said second spring innormally spaced relation to said one end of said iirst spring so as tobe thrust into engagement by said piston with said one end of said irstspring for establishing an electrical circuit from said plug to saidterminal when and only when said piston is unged towards the bottom ofsaid cylinder under the urging of said additional force.

2. In -a hydraulic clutch system including a set of clutch springs, thecombination comprising (a) a slave cylinder and a piston mounted torreciprocation in said slave cylinder,

(b) spring means coupled to said piston and having a predeterminedspring constant for urging said piston in its rest position toward thebottom of the cylinder with a predetermined force under normal operatingconditions, said piston in its rest position being additionally urgedtoward the bottom of the cylinder by the clutch springswhen the clutchis excessively worn or misadjusted,

(c) a iirst contact resiliently mounted within said cylinder at adistance from the bottom thereof,

(d) a second electrical contact,

(e) a spring resiliently supporting said second contact spaced from saidiirst contact between the head of said piston and said rst contact so asto be thrust into engagement by said piston with said `first contact,said spring having a spring constant such that its biasing force exceedsthe force of said biasing spring upon said piston so that the force ofsaid contact supporting spring will be overcome and the contacts willvengage only -whenzsaid piston is'- additionallyvurged towards thebottom of the cylinder by said clutch springs,

(i) means for electrically connecting one of said contacts to saidcylinder, `and l f (g) means for electrically connecting the other saidcontact t-o a point outside said cylinder. f 3. A hydraulic clutchsystem including means to excessive clutch disc wear comprising incombination (a) a friction disc and a pressure plate normally biasedinto engagement by springs,

A (b) a throwout bearing mounted on a shaft for movement towards andaway from said plate,

(c) la plurality of rockably mounted lingers f 1) anchored to said plateat one of their ends and having their other end normally held closelyspaced tfrom said bearing so that when said bearing is moved towardssaid pressure plate said fingers rock and separate said pressure platefrom said disc against the pressure of said springs,

(2) said lingers being subject to having their free ends levered by saidpressure plate against said bearing under the force of said springs whensaid `friction disc becomes excessively worn,

(d) la slave cylinder,

(e) a piston mounted for longitudinal movement in said cylinderand'having a head Ifor forming a pressure chamber with the bottom andwall portions of said cylinder,

(f) a source of pressurized fluid communicating-with said pressurechamber for urging said piston into an extended position away from thebottom of said cylinder,v i

(g) a connecting rod coupled to said piston,

(h) a rockably mounted arm operatively connected between said throw-outbearings and said piston connecting rod for forcing said bearing againstsaid tingers when said piston is in its extended position,

(i) a spring operatively coupled to said arm 4for rocking said arm so asto maintain said bearing out of contact with said fingers,

(j) said piston being normally subject to a predetermined biasing forceexerted by saidV spring through said arm and through said rod urgingsaid piston headv towards the bottom of said cylinder, said piston alsobeing subject to an additional force urging it towards the bottom ofsaid cylinder and exerted by said clutch springs through said fingers,said bearing, said arm, and said rod, when said clutch disc has becomeexcessively worn,

(ik) a iirst switch contact supported near the lbottom of said cylinder,

(l) a second switch contact,

(m) means for resiliently supporting said second contact between saidtirst contact and said piston head and acting in opposition to saidrocking spring so as to be thrust into engagement with said rtirstcontact by said piston head, said means having a stiliness requiring aforce in excess of said predetermined biasing .force to overcome it sothat said switch will close only when said additional force is exertedon said piston, indicating a worn clutch disc. f

4. lIn a hydraulic clutch system including a set of clutch springs, thecombination comprising (a) a slave cylinder having an access openingthrough its bottom wall and la piston mounted for reciprocation in saidslave cylinder,

(b) biasing means coupled to said piston and urging said piston in itsrest position toward the bottom l of the cylinder with a predeterminedforce under normal operating conditions, said piston inits'rest fposition being -additionally urged toward the bottom of the cylinder bythe clutch springs when the clutch is excessively worn or misadjusted,

(c) a plug seated in and sealing said cylinder opening,

(d) a first electrical contact on said plug,

(e) a second electrical contact located for movement toward said firstpiston contact in response to movement of said piston toward the bottomof said cylinder, i

(f) means for yieldably separating said second contact from said :Erstcontact, said separating means having a yield point which is higher thanthe force of said biasing means upon said piston, but lower than the sumof forces exerted by said biasing means and by said clutch springs uponsaid piston so that said separating means will yield and said contactswill engage only when said piston is addi- 1, tionally urged toward thebottom of the cylinder by said clutch springs, and

(g) means ffor electrically connecting said contact members in a commonelectric circuit outside said cylinder.

5. A hydraulic clutch system including means for detecting excessiveclutch disc wear comprising in combination (a) a friction disc and -apressure plate normally biased into engagement by springs,

(b) a throw-out bearing mounted on a shaft for movement towards and awayfrom said plate,

(c) a plurality of rockably mounted fingers (l) anchored to said plateat one of their ends and having their other end normally held closelyspaced from said bearing so that 'when said bearing is moved towardssaid pressure plate said fingers rock and separate said pressure platefrom said disc against the pressure of said springs,

(2) said fingers being subject to having their free ends levered by saidpressure plate against said bearing under the force of said springs whensaid friction disc becomes excessively worn,

(d) a slave cylinder,

(e) a piston mounted for longitudinal movement in said cylinder andhaving a head for forming a pressure chamber with the bottom and wallportions of said cylinder,

(f) a source of pressurized Huid communicating with said pressurechamber for urging said piston into an extended position away from thebottom of said cylinder,

(g) a connecting rod coupled to said piston,

(h) a rockably mounted arm operatively connected between said throw-outbearing and said piston connecting rod for forcing said bearing againstsaid lingers when said piston is in its extended position,

(i) biasing means operatively coupled to said arm yfor rocking said armso as to maintain said bearing out of contact with said lingers,

(j) said piston being normally subject to a predetermined biasing forceexerted by said biasing means through said arm and through said rodurging said piston head towards the bottom of said cylinder, said pistonalso being subject to an additional force urging it towards the bottomof said cylinder and exerted by said clutch springs through saidfingers, said bearing, said arm, vand said rod, when said clutch dischas become excessively worn,

(k) a first electrical contact near the bottom of said cylinder,

(l) a second electrical contact located for movement toward said firstcontact in response to movement of said piston toward the bottom of saidcylinder,

(m) means for yieldably separating said second contact from said firstcontact, said separating means having a yield point which is higher thansaid predetermined biasing force but lower than the sum of saidpredetermined force and said additional force so that said contacts willengage only when said v additional fonce is exerted on'said piston',indicating a worn clutch disc.

6. In a hydraulic clutch system including a set of 5 clutch springs, thecombination comprising (a) a slave cylinder and a piston mounted forreciprocation in said slave cylinder, i

(b) biasing means coupled to said piston and urging said piston in itsrest position toward the bottom of the cylinder with a predeterminedforce under normal operating conditions, said piston in its restposition being additionally urged toward the bottom of the cylinder bythe clutch springs -when the clutch is excessively worn or mis-adjusted,and

(c) force responsive means for conditionally conducting current mountednear the bottom of said cylinder and extending at least partially intosaid cylinder for operation by said piston as it approaches cylinderbottom, said means requiring for its operation a force which is greaterthan that exerted on said piston by said biasing means, but less thanthat exerted on said piston by said biasing means and said clutchsprings together, so that said means is operated by said piston when andonly when said piston is urged by the combined forces of said biasingmeans and said clutch spring, and

(d) means for connecting said switch in an electrical circuit outsidesaid cylinder.

7. A hydraulic clutch system including means for detecting excessiveclutch disc wear comprising in combination (a) a friction disc and apressure plate normally biased into engagement by springs,

(b) a throw-out bearing mounted on a shaft for movement towards `andaway from said plate,

(c) a plurality of rockably mounted lingers (1) anchored to said plateat one of their ends and having their other end normally held closelyspaced from said bearing so that when said bearing is moved towards saidpressure plate said iin-gers rock and separate said pressure plate fromsaid disc against the pressure of said springs,

(2) said lingers being subject to having their free ends levered by saidpressure plate lagainst said bearing under the force of said springswhen said friction disc becomes excessively worn,

(d) a slave cylinder,

(e) a piston mounted for longitudinal movement in said cylinder andhaving a head for forming a pressure chamber with the bottom and wallportions of said cylinder,

(f) a source of pressurized iluid communicating with said pressurechamber for urging said piston into an extended position away from thebottom of said cylinder,

(g) a connecting rod coupled to said piston,

(h) a rockably mounted arm connected between said throw-out bearing andsaid piston connecting rod for forcing said bearing againstsaid [lingerswhen said piston is in its extended position,

(i) biasing means coupled to said arm for rocking said arm so a-s tomaintain said bearing out of contact with said fingers,

(j) said piston being normally subject to a predetermined biasing forceexerted by said biasing means through said arm and through said rodurging said piston head towards the bottom of said cylinder, said pistonalso being subject to an additional force urging it towards the bottomof said cylinder and exerted by said clutch springs through saidfingers, said bearing, said arm, |and said rod, when said clutch dischas become excessively worn, and

(k) electrical means for detecting the additional force exerted by saidclutch springs on said piston, said Il f I I2 electrical means beingcharacterized in that it is 1,536,393 5/ 1925 Hellman. not actuated bythe normal separation of said fric- 2,323,489 7/1943 Schmidt et a1. tiondisc and said pressure plate. 2,852,757 9/ 1958 Fawick 20G-61.412,525,220 10/1950 Hague 192-91 References Cited 5 3,307,667 3/1967Maurice 192-91 UNITED STATES PATENTS E J M 535,264 3/1895 Smith. B` N AIN W WYCHE III Przmary Examzner

